In prior art full penetration welding using a backing material, the following procedures are usually taken. Firstly, a weld backing is formed from copper or a copper alloy with its length being identical to the length of the weld line of a workpiece to be welded and its configuration fitting to the shape of the weld line. Such a weld backing is held against the underside of the weld zone where a groove is formed and is fixed in position being supported by a number of supporting members. In this condition, a welding machine is moved along the weld line to weld throughout the entire length of the weld line. After completion of welding, the weld backing as well as its supporting members are removed.
However, the conventional weld backing made from copper or a copper alloy suffers from the following disadvantages: (1) the weld backing is too soft and therefore liable to wear; and (2) the penetration bead tends to stick to the weld backing in high temperature zones. For these reasons, the conventional weld backing is not reusable and cannot withstand long service. The present invention has been made in order to overcome such disadvantages and therefore an object of the invention is to provide a weld backing which is repeatedly usable in full penetration welding and can withstand long service.